With the rapid development of technology, the critical size of a transistor has already reached the nanometer range, and the number of transistors on a single chip is unbelievably large (reaching several billions transistors per chip). Metal wires play a very important role in chips because they connect different devices together in a single chip, or they interconnect different chips. However, due to the rapidly increasing number of transistors on a single chip and the increasingly complexity of chip design, the total length of metal wire needs to continuously increase, resulting in an increase in total resistance. Therefore, the signal delay and the power consumption of chips have become very critical challenges in today's semiconductor technology. Further, the capability of electrons to carry information is not as good as the photons. These factors result in an increased interest to utilize optical interconnection to replace metal wires to connect different devices and chips. The utilization of optical interconnections may lead to an increase in the capability to transmit huge volumes of information in the chips as well as lower power consumption. The rapid development of photonic components requires a high level integration so that different photonic components can form a system to realize a certain function. Consequently, silicon photonics has become a highly promising research field in the last several years.
Silicon photonics research also includes fundamental passive components such as waveguides. Integration of discrete photonic components into a single chip is a long-standing goal of integrated optics. In earlier reports of silica-on-silicon technology, the waveguide was formed in a silica layer by doping it with phosphorous (P) or germanium (Ge) atoms. An impressive level of integration, e.g. a 16×16 switch array on a single 6 inch silicon wafer, may be reached based on this technology. However, further increase in the integration density with this technology is restricted by the large minimal bending radius of silica waveguides, which is in the order of a few centimeters.
A significant step toward much denser integration has been demonstrated with silicon oxynitride (SiON) technology. A much higher index contrast is introduced between the core of the SiON waveguide and silica cladding, which allows the minimum bending radius to be reduced to below 1 mm. Further,
Aggressive scaling is introduced by silicon-on-insulator (SOI) technology, which involves forming the waveguide in a thin silicon layer. The extremely high refractive index contrast between the silicon core (n=3.5) and silica cladding layer (n=1.45) allows the waveguide core to be shrunk down to a submicron cross-section, while still maintaining single mode propagation at about 1.3 microns to about 1.5 microns telecommunications wavelengths. Such extreme light confinement allows the minimal bending radius to be reduced to the micron range.
SOI platform almost focuses on telecommunication wavelength which is about 1550 to about 2000 nm. However, silicon and germanium are transparent up to about 8 and about 15 μm, respectively, thus offering a range of applications in biochemical and environmental sensing, medicine, astronomy and communications. The major problem with a transition to the mid-infrared (MIR, about 2 μm to about 20 μm) is that SOI can be used only up to about 4 μm, due to the high absorption loss of silicon dioxide.
Therefore, alternative material platforms have to be utilized for longer wavelengths. Germanium-on-silicon (Ge-on-Si) platform has been proposed and demonstrated to be workable because the transparency of Ge extends to about 15 μm. Two micrometer thick strip Ge-on-Si waveguides with losses of about 3 dB/cm were first reported by Change et al. (“Low-loss germanium strip waveguides on silicon for the mid-infrared,” Opt. Lett., vol. 37, pp. 2883-2885, 2012). Similar losses for the same structure, at a similar wavelength range, were also reported by Malik et al. (“Germanium-on-silicon mid-infrared arrayed waveguide grating multiplexers,” IEEE Photon. Technol. Lett., vol. 25, no. 18, pp. 1805-1808, September 2013). These waveguides have been characterized to have a wavelength range of about 2 to about 2.6 μm.
Researchers at the University of Southampton have come up with a Ge-on-Si rib waveguide platform configured to carry light having a wavelength of about 3.8 μm. The waveguides were designed to have an etch depth of about 1.2 μm and a core width of about 2.25 μm. The propagation loss of these waveguides was measured by the cut back method giving a low loss of about 2.4±0.2 dB/cm at a wavelength of about 3.8 μm, which is claimed to be the lowest reported loss for Ge on Si waveguides structures (Mashanovich G Z et al. Silicon photonic waveguides and devices for near-and mid-IR applications, Selected Topics in Quantum Electronics, IEEE Journal of, 2015, 21(4): 1-12).
Ge-on-Si can extend the applicable wavelength to the mid-IR range. However, one challenge is that the refractive index contrast between Ge (n=4.1) and Si (n=3.4) is not larger than that of SOL As such, the radius of the bend part of the waveguides cannot be as small as a micron. Normally, a sensing chip is desired to be as small as possible so that the chip would not occupy large spaces in systems or devices. SOI cannot be used in sensing applications and Ge-on-Si cannot provide a compact design of waveguides.